a book in the Discovering Our Universe series by David Darling

3. The Icy Giants

As we move farther from
the Sun, we leave behind the warm, rocky worlds of the Solar System. We
enter a place where the Sun's heat and light is not so strong; where space
is cold, and the planets are frozen. We enter the realm of the icy giants.

Jupiter – the Super-Planet

Jupiter

At a distance of 484 million miles (778 million kilometers) from the Sun,
we come to the first of these giants. It is mighty Jupiter, the super-planet.

Within its huge body – 88,378 miles (142,200 kilometers) in diameter
– Jupiter could hold more than 1,300 Earths. It outweighs, by 2½
times, all the others planets combined. And its powerful pull of gravity
can be felt throughout the Sun's kingdom.

When we look at Jupiter, we do not see a firm surface. Instead we see the
brown, yellow, and orange cloud-tops of an enormously thick atmosphere.
Because Jupiter spins very quickly, these clouds are drawn out into dark
and light bands. The spinning planet causes the cloud bands to swirl and
to form raging whirlpools. The most fantastic of Jupiter's storms, the Great
Red Spot, is a centuries-old cyclone big enough to swallow Earth.

If you can imagine a star that has been frozen, you'll have some idea of
what Jupiter is like inside. It's made mostly of hydrogen – the lightest
of all elements. It also contains some helium, and a dash of other chemicals
such as ammonia, methane, and ice.

In the cloud-tops of Jupiter, the hydrogen takes the form of a gas. It's
mixed with some rarer substances that give Jupiter's belts and spots their
beautiful "fall" colors. Deeper down, layers of ammonia crystals, ice crystals,
and water droplets appear. Hundreds of miles down, the gassy atmosphere
of Jupiter changes to a liquid. This vast sea of liquid hydrogen is lit
only by the glow of giant lightning bolts.

Most of Jupiter is filled by this gloomy underground hydrogen sea. At great
depths, though, the hydrogen is so squashed that it may behave like a liquid
metal. The center of Jupiter is probably a rocky ball, somewhat bigger than
Earth. This solid core is wrapped in the blanket of the planet's immense
atmosphere.

Like the Sun, Jupiter actually gives off more heat than it receives. At
-240°F (-150°C), its cloud-tops are very cold, but the heat from
within makes them warmer than they would otherwise be. Deep inside the atmosphere,
the temperatures are much higher.

Saturn – the Beautiful Giant

Saturn

In recent years scientists were surprised to discover that thin rings surround
the planets Jupiter, Uranus, and Neptune. For a long time, only one planet
was known to have rings. This was Saturn, the most beautiful of the icy
giants.

Saturn, smaller and cooler than Jupiter, is 74,145 miles (119,300 kilometers)
in diameter and 887 million miles (1.43 billion kilometers) from the Sun.
Like Jupiter, it spins rapidly – once every 10 hours, 39 minutes compared
with Jupiter's 9 hours, 55 minutes. And like Jupiter, this fast spin makes
it a flattened, rather than a perfectly round, ball.

Saturn stands out among the planets because of the size and majesty of its
rings. These stretch like a giant halo, 169,200 miles (273,300 kilometers)
across, around Saturn's yellow, banded globe. Although the rings appear
to be solid, they are in fact made of countless small, icy rocks that circle
Saturn like tiny moons. The rings are also amazingly thin – less than
a mile in depth!

Of all the planets, Saturn is the lightest for its size. Standing on Saturn's
surface, you would weigh less than you do on Earth. And if the whole of
Saturn could be dropped into a vast ocean of water, it would float!

Uranus and Neptune – the Distant Giants

Neptune

Beyond Saturn lie the two smaller icy giants of the Solar System, Uranus
and Neptune. Like monstrous twins, both are about 30,000 miles (48,000 kilometers)
in diameter. However, Uranus, at a distance of 1.78 billion miles (2.87
billion kilometers) is more than a billion miles closer to the Sun. Both
planets were visited in the late 1980s by the Voyager 2 probe,
which earlier had flown past Jupiter and Saturn.

Uranus and Neptune, like Jupiter and Saturn, have small rocky cores surrounded
by very thick layers of frozen gases. Because of their great distances from
the Sun, though, they are even colder than their giant neighbors. Uranus's
surface shivers at -350°F (-212°C). Neptune's stays at a frosty
-360°F (-218°C).

Uranus is special because it's the only world "lying on its side." While
the other planets spin like upright tops as they go around the Sun, Uranus
spins like a top that has tipped over. For us this motion would make Uranus's
year – lasting 165 Earth-years – and its seasons appear very
strange, indeed.

In 1977, scientists learned that Uranus is surrounded by several rings.
These are much narrower than those of Saturn and have bigger gaps between
them. Neptune too has been shown to be a ringed world. Rings around planets,
it seems, are not so rare as they were once thought to be.

Children of the Icy Giants

Io

If scientists were surprised to discover new rings around Jupiter, Uranus,
and Neptune, they were astonished to learn about the many moons of the icy
giants. The various spacecraft that have flown to the outer Solar System
have opened our eyes to the remarkable families of mini-worlds that circle
these great planets.

Jupiter has several of the biggest moons in the Solar System. Two of them
– Ganymede and Callisto – are even larger than Mercury.

Another of Jupiter's moons, Io, would surely take the prize as the strangest
world in the Sun's kingdom. Its entire surface is covered by orange chemicals
thrown out by volcanoes. In fact, during the Voyager missions, several volcanoes
were seen shooting great fountains of molten material into Io's sky!

Callisto, and a fourth big satellite of Jupiter, Europa, show just how different
two neighboring worlds can be. Callisto is battered and pockmarked by densely
packed craters. Europa, on the other hand, has a strange, ice-cracked surface
that, from a distance, looks as smooth as a billiard ball.

In addition to Ganymede, Callisto, Io, and Europa, Jupiter has 59 other
satellites, mostly quite small. Saturn, too, has a big collection of moons
– a total of 60 at the latest count.

Saturn's largest moon, Titan, is blanketed by a thick orange haze. At 3,200
miles (5,150 kilometers) in diameter, it is almost as big as Ganymede. What's
more it is the only moon in the Solar System known to have a thick atmosphere.

Saturn's other moons are all fairly small, but still interesting. Iapetus
has one side coated by ice and the other made of dark, bare rock. Mysterious
Mimas has a crater that looks like a giant eye.

Uranus and Neptune also have families of moons. Neptune's big moon Triton
is especially fascinating because it has ice volcanoes that shoot substances
such as frozen nitrogen and methane several miles above the surface.